1. Technical Field
The present invention relates to needle coke useful for applications including forming graphite electrodes. More particularly, the present invention relates to a process for producing needle coke exhibiting reduced puffing characteristics from a coal tar starting material. The invention also includes the reduced puffing needle coke.
2. Background Art
Carbon electrodes, especially graphite electrodes, are used in the steel industry to melt both the metals and supplemental ingredients used to form steel in electrothermal furnaces. The heat needed to melt the substrate metal is generated by passing current through a plurality of electrodes and forming an arc between the electrodes and the metal. Currents in excess of 100,000 amperes are often used.
Electrodes are typically manufactured from needle coke, a grade of coke having an acicular, anisotropic microstructure. For creating graphite electrodes that can withstand the ultra-high power throughput, the needle coke must have a low electrical resistivity and a low coefficient of thermal expansion (CTE) while also being able to produce a relatively high-strength article upon graphitization.
The specific properties of the needle coke may be dictated through controlling the properties of the coking process in which an appropriate carbon feedstock is converted into needle coke. Typically, the grade-level of needle coke is a function of the CTE over a determined temperature range. For example, premium needle coke is usually classified as having an average CTE of from about 0.00 to about 0.30×10−6/C° over the temperature range of from about 30° C. to about 100° C. while regular grade coke has an average CTE of from about 0.50 to about 5.00×10−6/C° over the temperature range of from about 30° C. to about 100° C.
To evaluate the CTE of a coke, it is first calcined to a temperature of about 1,000 to 1,400° C. It is then admixed with a molten pitch binder and the pitch/coke mixture is extruded to form a green electrode. The electrode is then baked to about 800-900° C. and then heated from 2,800-3,400° C. to effect graphitization. The CTE is measured on the graphitized electrode using either a dilatometer or the capacitance method (the capacitance method is described in a publication titled “Capacitance Bridge Measurements of Thermal Expansion” presented at the 1986 International Carbon Conference in Baden-Baden Germany. The procedure for evaluating coke CTE is found in publication by E. A. Heintz, Carbon Volume 34, pp. 699-709 (1996), which are incorporated herein by reference in their entirety).
In addition to low CTE, a needle coke suitable for production of graphite electrodes preferably has a very low content of sulfur and nitrogen. Sulfur and nitrogen in the coke generally remain after calcination and are only completely removed during the high temperature graphitization process.
If the needle coke contains too great a degree of nitrogen or sulfur, the electrode will experience “puffing” upon graphitization. Puffing is the irreversible expansion of the coke particles which creates cracks or voids within the electrode, diminishing the electrode's structural integrity as well as drastically altering both its strength and density.
The degree of puffing generally correlates to the percentage of nitrogen and sulfur present in the needle coke. Both the nitrogen and sulfur atoms are bonded to the carbon within the feedstock through covalent bonding typically in a ring arrangement. The nitrogen-carbon and sulfur-carbon bonding is considerably less stable than carbon-carbon bonding in high temperature environments and will rupture upon heating. This bond rupture results in the rapid evolution of nitrogen and sulfur-containing gases during high temperature heating, resulting in the physical puffing of the needle coke. Another source of puffing may also involve the rupture of sulfur to sulfur bonds.
A variety of methods have been attempted to reduce the puffing of needle coke during the graphitization process, with most of the focus directed to the effects of sulfur. The approaches used involve either treating the needle coke feedstock with a catalyst and hydrogen to remove sulfur prior to coking or to introduce chemical additives to the coke which inhibit the puffing process.
One such approach has been the use of an inhibitor additive to either the initial feedstock or the coke mixture prior to the graphitization to an electrode body. U.S. Pat. No. 2,814,076 teaches of the addition of an alkali metal salt to inhibit the puffing. Such salts are added immediately prior to graphitizing an electrode. Notably, sodium carbonate is added by impregnating the article through a sodium carbonate solution.
U.S. Pat. No. 4,312,745 also describes the use of an additive to reduce the puffing of sulfur-containing coke. Iron compounds, such as iron oxide are added to the sulfur-containing feedstock with the coke being produced through the delayed-coking process. However, this process can increase the CTE of the coke.
Orac et al. (U.S. Pat. No. 5,118,287) discloses the addition of an alkali or alkaline earth metal to the coke at a temperature level above that where the additive reacts with the carbon but below the puffing threshold to thereby preclude puffing.
Jager (U.S. Pat. No. 5,104,518) describes the use of sulphonate, carboxylate or phenolate of an alkaline earth metal to a coal tar prior to the coking step to reduce nitrogen puffing in the 1400° C.-2000° C. temperature range. Jager et al. (U.S. Pat. No. 5,068,026) describes using the same additives to a coke/pitch mixture prior to baking and graphitization, again to reduce nitrogen-based puffing.
Other attempts have been made to preclude the puffing of electrodes through the use of carbon additives or various hydro-removal techniques. In U.S. Pat. No. 4,814,063, Murakami et al. describes the creation of an improved needle coke through the hydrogenation of the starting stock in the presence of a hydrogenation catalyst. Subsequently, the hydrogenated product undergoes thermal cracking with the product being cut into different fractions. In Japan Patent Publication 59-122585, Kaji et al. describes hydrorefining a pitch in the presence of a hydrogenating catalyst to remove nitrogen and sulfur, followed by coking of the pitch to give a reduced puffing needle coke.
Goval et al. (U.S. Pat. No. 5,286,371) teaches of passing a feedstock through a hydrotreating reaction zone to produce a hydrotreated residual product wherein the product can undergo a solvent extraction process.
Didchenko et al. (U.S. Pat. No. 5,167,796) teaches the use of a large pore size hydrotreating catalyst with hydrogen to remove sulfur from a petroleum decant oil prior to coking.
Unfortunately, needle coke produced by the prior art usually fails to address the problems of nitrogen remaining in the needle coke that is to be graphitized into an electrode. The additives used to reduce the puffing characteristics of needle coke counteract the sulfur components which would otherwise be liberated from the needle coke but fail to preclude puffing resulting from the nitrogen components. Since nitrogen puffing is not controlled, the use of such additives can result in a finished electrode product of inferior quality as the electrode will likely possess both a lower density and a lower strength. The addition of chemicals to the coke feedstocks or to the pitch can lead to the presence of solids during mesophase formation which could raise the CTE of the derived coke. Furthermore, hydrogenation processes require a significant energy input as high temperatures needed for extended heat treatments to remove a substantial amount of nitrogen from the feedstock. Furthermore, hydrogen must be applied for the hydrogenation and accompanying removal of the sulfur and nitrogen from the feedstock.
What is desired, therefore, is a process for producing reduced puffing needle coke which does not require the use of puffing inhibitor additives and therefore does not decrease the strength and density of the final electrode. Furthermore, a process is desired requiring less thermal energy for the removal of nitrogen from the feedstock as well as no input stream of hydrogen. Indeed, a process which is superior in removing nitrogen from a feedstock for the production of needle coke and/or binder pitch for producing a graphitized electrode article has been found to be necessary for producing high strength, reduced-puffing electrodes. Also desired is the inventive reduced-puffing needle coke with reduced nitrogen content for the production of graphite electrodes. Furthermore, since coal-based feedstocks for needle coke, such as coal tar, have high nitrogen contents, a process is desired which would allow the use of such feedstocks for production of needle cokes suitable for high performance graphite electrodes.